1. Field of the Invention
This invention relates to an optical disc having a substrate and a recording layer and a light transmitting layer formed in this order thereon and which is adapted for recording and/or reproducing information signals on the recording layer by the light incident on the light transmitting layer.
2. Description of the Related Art
Among optical recording mediums for audio or video, adapted for recording the various information thereon, there are, for example, an optical disc on which information signals are pre-recorded by embossed pits, a phase-change disc on which information signals are written by exploiting phase changes of the recording film, and a magneto-optical disc on which information signals are written by exploiting the photomagnetic effect of the recording film. With this type of the optical recording medium, a recording layer a light reflecting layer and a protective layer are sequentially formed on a transparent substrate, and the laser light is adapted to fall from the transparent substrate side for recording and/or reproducing information signals on the recording layer.
Meanwhile, with the conventional optical disc, the recording density of the optical disc is determined by the minimum spot diameter of a laser light source employed. That is, the smaller the laser spot diameter, the higher is the recording density that can be achieved. This laser spot diameter is proportionate to the xcex/NA of the of the recording/reproducing optical system, where xcex is the wavelength of the laser light and NA is the numerical aperture of an objective lens. Thus, if desired to realize high recording density of the optical disc, the wavelength xcex of the laser light needs to be reduced to increase the numerical aperture of the objective lens.
However, if the numerical aperture of the objective lens is increased, coma aberration poses a problem, since coma aberration is proportionate to [skew angle, that is the tilt angle of the objective lens to the optical axis of the optical disc]xc3x97NA3xc3x97[thickness of the optical disc traversed by the laser light]. For tackling with this problem of coma aberration, a method of reducing the thickness of the transparent substrate, as a disc substrate, is under consideration.
However, for the transparent substrate of the optical disc, an injection-molded substrate of plastics, produced by the injection molding method, is predominantly used. It is technically difficult to fabricate the injection-molded substrate to an extremely thin thickness and to high precision. The method for improving the recording density of the conventional optical disc by reducing the wavelength xcex of the laser light or by enlarging the numerical aperture of the objective lens is, as it were, approaching a technically unsurmountable limit level.
The present inventors have proposed an optical disc in which, for possibly improving the recording density further, a light reflecting layer, a recording layer and a light transmitting layer are sequentially formed on a substrate and the laser light is caused to fall from the light transmitting layer to record and/or reproduce information signals on a signal recording area of the signal recording layer.
In particular, since the optical disc is fabricated by sequentially forming the light reflecting layer, recording layer and the light transmitting layer on the substrate, the light transmitting layer on which falls the laser light can be manufactured to a thin thickness and to high accuracy. The result is that the optical disc can sufficiently cope with the high NA of the objective lens to improve the recording density.
The methods for forming the light transmitting layer on the optical disc may be exemplified by a first method of sequentially forming a light reflecting layer and a recording layer on a substrate and subsequently affixing a resin sheet 100 xcexcm in thickness on the recording layer via a transparent adhesive layer several xcexcm in-between to form a light transmitting layer formed by the resin sheet, and a second method of dripping a UV curable resin on the recording layer and stretching the UV curable resin on rotation and illuminating the UV rays on the substrate kept in a stationary state to cure the resin to form the light transmitting layer.
With the first method, the light transmitting layer exhibits optimum thickness evenness. However, this light transmitting layer is difficult to manufacture with respect to handling of the resin sheet and double refraction and hence it has scarcely been put to practical use.
With the second method, a center opening in the substrate is temporarily stopped and, in this state, a UV curable resin is dripped onto this center hole and stretched on rotation. The substrate is then kept in a stationary state and irradiated with the UV rays to cure the UV curable resin to form the light transmitting layer. Since this second method represents further development of the technique of forming a protective film for a conventional optical disc, such as compact disc (CD), and hence the stock handling, know-how in coating and designing of the apparatus so far developed can be utilized, it lends itself to mass production.
However, with this second method, in which the UV curable resin is stretched on rotation, the resin tends to be moved towards the outer rim during the stretching process under the centrifugal force, so that a protuberant portion tends to be produced in an outer rim portion. Moreover, with this second method, since the UV rays are illuminated on the substrate 101 coated with the UV curable resin by rotational stretching, as the substrate is kept stationary, the resin deposited on the outer rim portion is raised and reverted towards the inner rim side under the surface tension to form a humped portion 100 of a sizeable width, as a result of which the light transmitting layer 102 presents a hump 100 on its outer rim portion.
If the UV curable resin is coated to a film thickness of 100 xcexcm on the substrate of the same size as the conventional CD, with the substrate being then rotated for stretching the resin and then being halted to illuminate the UV rays to form the light transmitting layer thereon, there is formed a humped portion in the outer rim area having a radial width of 5 mm or more, thus narrowing the signal recording area.
If the humped portion having a width sufficient to affect the signal recording area is formed in this manner on the outer rim area of the light transmitting layer, a sufficient signal recording area cannot be obtained to render it impossible to improve the recording capacity or to lead possibly to reduced recording capacity. Specifically, for realizing the high recording capacity, it is necessary to procure a signal recording area comparable to that of the CD or DVD. In order to achieve this, the humped portion formed on the outer rim of the light transmitting layer needs to be of a width of not larger than 1.5 mm.
It is therefore an object of the present invention to provide an optical disc that can be increased further in recording capacity and that has a light transmitting layer of optimum surface properties free to the utmost extent from humps on its outer rim portion, and method for manufacturing such optical disc.
In one aspect, the present invention provides an optical disc in which a recording layer and a light transmitting layer are sequentially formed on a substrate and in which light is incident from the side of the light transmitting layer to record and/or reproduce information signals for a signal recording area of the recording layer, wherein a radial distance from the outermost area of the substrate to signal recording area is selected to be larger than a radial width of a hump produced on the outer rim of the light transmitting layer and wherein the height of hump from the surface of the light transmitting layer is 70 xcexcm or less.
The optical disc of the present invention, in which the width and the height of a hump formed at an outer rim of the light transmitting layer following the manufacture of the disc are limited as described above, has optimum surface properties by having the light transmitting layer free from surface irregularities as far as possible to provide a signal recording area as broad as possible.
In another aspect, the present invention provides a method for producing an optical disc in which a recording layer and a light transmitting layer are sequentially formed on a substrate and in which light is incident from the side of the light transmitting layer to record and/or reproduce information signals, including the steps of coating, at the time of forming the light transmitting layer, a UV curable resin on the portion of the substrate carrying recording layer and rotating the substrate carrying the UV curable resin coated thereon and concurrently illuminating UV rays on UV curable resin to cure UV curable resin.
With the optical disc manufacturing method according to the present invention, UV rays are illuminated on the substrate, carrying the UV curable resin, as the disc is run in rotation, thus minimizing the risk of the UV curable resin on the outer rim of the substrate reverting to the inner rim under centrifugal force to produce a hump.